Field
The present invention relates to a method for measuring a mass flow rate in a thermal type mass flow meter, a thermal type mass flow meter which uses the method, and a thermal type mass flow controller which uses the thermal type mass flow meter.
Background
A mass flow meter is widely used for the purpose of measuring a mass flow rate of a process gas supplied in a chamber in a manufacturing process of a semiconductor, for example. In addition, a mass flow meter not only is used independently as mentioned above, but also is used as a part which constitutes a mass flow control device (mass flow controller) in combination with other parts, such as a flow control valve and a control circuit. Although there are various types of mass flow meters in the art, a thermal type mass flow meter is widely used since it can accurately measure a mass flow rate of a fluid (for example, a gas and a liquid) with a comparatively simple configuration. Among thermal type mass flow meters, especially, a capillary heating type thermal type mass flow meter is used widely.
Generally, a capillary heating type thermal type mass flow meter is constituted by a passage through which a fluid flows, a bypass (may be referred to as a “flow element” or a “laminar-flow element”) prepared in the middle of the passage, a sensor tube (may be referred to as a “capillary” or a “capillary tube”) which branches from the passage on an upstream side of the bypass and joins the passage again in a downstream side of the bypass, a pair of sensor wires which is wound around the sensor tube and a sensor circuit which comprises a bridge circuit containing the sensor wires and other resistive elements (see for example, Japanese Patent Application Laid-Open “kokai” No. 2009-192220 official report). The bypass is configured so that it has a flow resistance against a fluid and a fixed proportion of a fluid which flows through the passage branches to the sensor tube.
In the above-mentioned configuration, when the pair of sensor wires is made to generate heat by applying a predetermined electric voltage (or a predetermined electric current), heat generated from the sensor wires is removed (drawn) by a fluid which flows through the sensor tube. As a result, the fluid which flows through the sensor tube is heated. In this case, the sensor wire on the upstream side has its heat removed by the fluid which is not yet heated. On the other hand, the sensor wire on the downstream side has its heat removed by the fluid which has been already heated with the sensor wire on the upstream side. For this reason, the heat removed from the sensor wire on the upstream side is larger than the heat removed from the sensor wire on the downstream side. As a result, temperature of the sensor wire on the upstream side becomes lower than temperature of the sensor wire on the downstream side. For this reason, an electrical resistance value of the sensor wire on the upstream side becomes lower than an electrical resistance value of the sensor wire on the downstream side. A difference between the electrical resistance values resulting from thus produced temperature difference between the sensor wire on the upstream side and the sensor wire on the downstream side becomes larger, as a mass flow rate of the fluid which flows through the sensor tube becomes larger.
A change of the difference in the electrical resistance value between the sensor wire on the upstream side and the sensor wire on the downstream side according to a mass flow rate of the fluid as mentioned above can be detected as a change of a potential difference by using a bridge circuit, etc., for example. Furthermore, this potential difference can be detected, for example, as an output signal outputted as a voltage value or a current value through an operational amplifier, etc. Based on the output signal thus detected, a mass flow rate of the fluid which flows through the sensor tube can be obtained, and a mass flow rate of the fluid which flows through the passage can be obtained based on the mass flow rate of the fluid which flows through the sensor tube (will be mentioned later in detail).
For example, in a thermal type mass flow meter which has a configuration as mentioned above, a certain specific fluid (for example, nitrogen gas (N2)) is flowed at a certain mass flow rate, used as a reference, (for example, a maximal flow rate (full scale) of the thermal type mass flow meter), and a reference signal intensity which is an intensity (a voltage value or a current value) of the above-mentioned output signal at this time is measured previously. Then, when a mass flow rate of the above-mentioned specific fluid is measured by the thermal type mass flow meter, an actual measured signal intensity which is an intensity of the above-mentioned output signal at this time of actual measurement is measured, and a mass flow rate of the above-mentioned specific fluid is calculated based on a proportion of this actual measured signal intensity to the above-mentioned reference signal intensity.
However, in fact, when measuring a mass flow rate of a fluid which has different thermal physical properties (for example, heat capacity, etc.) from the above-mentioned specific fluid, it is difficult to accurately calculate a mass flow rate based on the proportion of an actual measured signal intensity to the reference signal intensity as mentioned above. Therefore, in the art, it is known to accurately calculate a mass flow rate of a fluid which has different thermal physical propertied from the above-mentioned specific fluid by correcting a mass flow rate with a conversion coefficient which is referred to as a conversion factor (CF), for example.
However, in fact, a mass flow rate of a fluid may be measured at different temperature and/or pressure from a condition under which the reference signal intensity is measured. In the case of an ideal gas, its thermal physical property is constant even at different temperature and/or pressure from a condition under which the reference signal intensity is measured. Moreover, also as for fluids which show behavior close to that of an ideal gas, such as noble gases (rare gases) (Ar, etc.) and nitrogen gas (N2), for example, since their thermal physical properties are approximately constant, it is substantially possible to accurately calculate a mass flow rate thereof based on the proportion of an actual measured signal intensity to the reference signal intensity as mentioned above.
However, many fluids show different behavior from that of an ideal gas. Specifically, thermal physical property of many fluids changes with temperature and/or pressure. Therefore, in order to accurately measure a mass flow rate of such a fluid, it is necessary to calculate a mass flow rate in consideration of not only its thermal physical property, but also a dependency of its thermal physical property on temperature and/or pressure. Therefore, in the art, various approaches for accurately measuring a mass flow rate of a fluid showing a different behavior from that of an ideal gas by taking into consideration a kind, temperature and pressure of the fluid whose mass flow rate is to be measured has been proposed.
For example, a technology for calculating an accurate mass flow rate using a value which is obtained by correcting a known physical property value (heat capacity) of a gas whose mass flow rate is to be measured according to temperature of the gas has been proposed (see for example, Japanese Patent Application Laid-Open “kokai” No. H03-204705 official report). Moreover, a technology for calculating an accurate mass flow rate using values which are obtained by correcting gas coefficients predetermined for every kinds and set flow rates of a gas whose mass flow rate is to be measured according to pressure of the gas has been proposed (see for example, Japanese Patent Application Laid-Open “kokai” No. 2010-091320 official report and Japanese Patent Application Laid-Open “kokai” No. 2010-169657 official report). Furthermore, a technology for calculating an accurate mass flow rate using correction coefficients predetermined for every kinds, temperatures and pressures of the gas whose mass flow rate is to be measured has been proposed (see for example, Japanese Patent Application Laid-Open “kokai” No. 2009-087126 official report).
In accordance with these technologies, in consideration of not only a kind of a gas whose mass flow rate is to be measured, but also temperature and/or pressure of the gas when measuring the mass flow rate, mass flow rates of various gases can be measured more accurately.